|Crop Knowledge Master||Fungi|
|wilts, rots, stem decay...|
Andre K. Gonsalves, Educational Specialist
Stephen A. Ferreira, Extension Plant Pathologist
Department of Plant Pathology, CTAHR
University of Hawaii at Manoa
In Hawaii, hosts of Fusarium oxysporum include: potato, sugarcane, garden bean, cowpea, Prickly pear, cultivated zinnia, pansy, Assam rattlebox, Baby's breath, and Musa sp. (Raabe et al., 1981).
Like various other plant pathogens, Fusarium oxysporum has several specialized forms - known as formae specialis (f.sp.) - that infect a variety of hosts causing various diseases. In Hawaii, these include: Fusarium oxysporum f.sp. asparagi (fusarium yellows on asparagus); f.sp. callistephi (wilt on China aster); f.sp. cubense (Panama disease/wilt on banana); f.sp. dianthi (wilt on carnation); f.sp. koae (on koa); f.sp. lycopersici (wilt on tomato); f.sp. melonis (fusarium wilt on muskmelon); f.sp. niveum (fusarium wilt on watermelon); f.sp. pisi (on edible-podded pea); f.sp. tracheiphilum (wilt on Glycine max); and f.sp. zingiberi (fusarium yellows on ginger) (Raabe et al., 1981).
Overall, the distribution of Fusarium oxysporum is known to be COSMOPOLITAN. However, the different special forms (f.sp.) of F. oxysporum often have varying degrees of distribution.
Fusarium oxysporum and its various formae speciales have been characterized as causing the following symptoms: vascular wilt, yellows, corm rot, root rot, and damping-off. The most important of these is vascular wilt. Of the vascular wilt-causing Fusaria, Fusarium oxysporum is the most important species (Agrios, 1988; Smith et al., 1988). Strains that are rather poorly specialized may induce yellows, rot, and damping-off, rather than the more severe vascular wilt (Smith et al., 1988).
Since fusarium wilt is the most important disease caused by F. oxysporum, the focus of this section will be on this symptom. In general, fusarium wilts first appear as slight vein clearing on the outer portion of the younger leaves, followed by epinasty (downward drooping) of the older leaves. At the seedling stage, plants infected by F. oxysporum may wilt and die soon after symptoms appear. In older plants, vein clearing and leaf epinasty are often followed by stunting, yellowing of the lower leaves, formation of adventitious roots, wilting of leaves and young stems, defoliation, marginal necrosis of remaining leaves, and finally death of the entire plant (Agrios, 1988). Browning of the [vascular tissue] is strong evidence of fusarium wilt. Further, on older plants, symptoms generally become more apparent during the period between blossoming and fruit maturation (Jones et al., 1982; Smith et al., 1988).
In solid media culture, such as potato dextrose agar (PDA), the different special forms of F. oxysporum can have varying appearances. In general, the aerial mycelium first appears white, and then may change to a variety of colors - ranging from violet to dark purple - according to the strain (or special form) of F. oxysporum. If sporodochia are abundant, the culture may appear cream or orange in color (Smith et al., 1988).
F. oxysporum produces three types of asexual spores: microconidia, macroconidia, and chlamydospores (Agrios, 1988). [Microconidia] are one or two celled, and are the type of spore most abundantly and freqeuntly produced by the fungus under all conditions. It is also the type of spore most frequently produced within the vessels of infected plants. [Macroconidia] are three to five celled, gradually pointed and curved toward the ends. These spores are commonly found on the surface of plants killed by this pathogen as well as in sporodochialike groups. [Chlamydospores] are round, thick-walled spores, produced either terminally or intercalary on older mycelium or in macroconidia. These spores are either one or two celled (Agrios, 1988).
F. oxysporum is an abundant and active saprophyte in soil and organic matter, with some specific forms that are plant pathogenic (Smith et al., 1988). Its saprophytic ability enables it to survive in the soil between crop cycles in infected plant debris. The fungus can survive either as mycelium, or as any of its three different spore types (Agrios, 1988).
Healthy plants can become infected by F. oxysporum if the soil in which they are growing is contaminated with the fungus. The fungus can invade a plant either with its sporangial germ tube or mycelium by invading the plant's roots. The roots can be infected directly through the root tips, through wounds in the roots, or at the formation point of lateral roots (Agrios, 1988). Once inside the plant, the mycelium grows through the root cortex intercellulary. When the mycelium reaches the xylem, it invades the vessels through the xylem's pits. At this point, the mycelium remains in the vessels, where it usually advances upwards toward the stem and crown of the plant. As it grows, the mycelium branches and produces microconidia, which are carried upward within the vessel by way of the plant's sap stream. When the microconidia germinate, the mycelium can penetrate the upper wall of the xylem vessel, enabling more microconidia to be produced in the next vessel. The fungus can also advance laterally as the mycelium penetrates the adjacent xylem vessels through the xylem pits (Agrios, 1988).
Due to the growth of the fungus within the plant's vascular tissue, the plant's water supply is greatly affected. This lack of water induces the leaves' stomata to close, the leaves wilt, and the plant eventually dies. It is at this point that the fungus invades the plant's parenchymatous tissue, until it finally reaches the surface of the dead tissue, where it sporulates abundantly (Agrios, 1988). The resulting spores can then be used as new inoculum for further spread of the fungus.
F. oxysporum is primarily spread over short distances by irrigation water and contaminated farm equipment. The fungus can also be spread over long distances either in infected transplants or in soil. Although the fungus can sometimes infect the fruit and contaminate its seed, the spread of the fungus by way of the seed is very rare (Agrios, 1988). It is also possible that the spores are spread by wind.
Because F. oxysporum and its many special forms affect a wide variety of hosts, the management of this pathogen is discussed in more detail in the respective summaries. In general, some effective means of controlling F. oxysporum include: disinfestation of the soil and planting material with fungicidal chemicals, crop rotation with non-hosts of the fungus, or by using resistant cultivars (Jones et al., 1982; Agrios, 1988; Smith et al., 1988).
Agrios, G.N. 1988. Plant Pathology, 3rd. ed. Academic Press, Inc.: New York. 803pp.
Farr, D.F., G.F. Bills, G.P. Chamuris, and A.Y. Rossman, eds. 1989. Fungi on plants and plant products in the United States. APS PRESS: St. Paul. 1252pp.
Jones, J.P., J.B. Jones, and W. Miller. 1982. Fusarium wilt on tomato. Fla. Dept. Agric. & Consumer Serv., Div. of Plant Industry. Plant Pathology Circular No. 237.
Raabe, R.D., I.L. Conners, and A.P. Martinez. 1981. Checklist of plant diseases in Hawaii: including records of microorganisms, principally fungi, found in the state. Hawaii Institute of Tropical Agriculture and Human Resources (CTAHR), Information Text Series 022. 313pp.
Smith, I.M., J. Dunez, D.H. Phillips, R.A. Lelliott, and S.A. Archer, eds. 1988. European handbook of plant diseases. Blackwell Scientific Publications: Oxford. 583pp.